The goal is to use contemporary electrophysiologic and molecular techniques to characterize remodeling of the atrial substrate required for the establishment of sustained atrial fibrillation (AF) and flutter (AFL) in the canine sterile pericarditis model. Hypotheses to be tested include: # 1) a time course of atrial pathophysiologic changes (electrophysiologic and cellular) in response to pericarditis leads to development of (a) first inducibility of a reentrant circuit of very short cycle length that produces AF; later a line of functional block between the venae cavae which permits the development of AFL; and still later neither; all of which are related to alterations in the distribution and functional properties of cardiac gap junctions; (b) first a loss of connexins, then a loss of myocytes with their subsequent replacement by fibroblasts in the outer layers of the atrial myocardium, but no changes in the endocardial layer; (c) a decrease in the atrial epicardial wavelength associated with inducibility of AF or AFL; # 2) the endocardial atrial effective refractory period and conduction time will remain constant throughout the time course of the study. Data from the proposed studies will lead to an improved basis for prevention and treatment of AF and AFL. [unreadable] [unreadable] [unreadable]